Mae 前 means "front". Keri 蹴り (it is spelled with a "G" when another word preceeds it), means "kick". The kanji Mae can also be pronounced as "Zen", like Zenkutsu dachi or Zenshinshite.
Mae tells you something about the direction of the technique. Although itis a subject for a different blog altogether, it is inconsistent where names of techniques come from. In case of Maegeri, it is about the direction of the technique, while Sokuto for example is about the part of the foot that is used to kick. Another one is Sunegeri, that name has to do with the target.
This headline is actually not so appropriate, since correct depends on the situation and is therefore not a fixed answer. Furthermore, Wado always has a kind of leeway, as Sakagami sensei calls it. In other words, a margin. The strict thinking does not suit Wado thinking.
For basic training, your foot should be streched with your toes pointing up. This implies the use of the ball of the foot.
The necessity of streching the foot, or actually increasing the angle of your foot compared to your lower leg, depends on the angle of the kick and the position of the target. If your opponent is leaning forward, you have to pull up your foot if you wish to use the ball of your foot for attack. This also applies to kicking lower targets such as the shin.
If you execute Maegeri with the instep in the groin, the kick is formally called Kingeri. Another variation is Sunegeri. Sunegeri is a Maegeri with the ball of the foot, that is aimed to the shin of the opponent. Sune means shin, therefore it is called Sunegeri.
Maegeri has another variation which is called Maegeri kekomi. Kekomi means to thrust and can be done with the heel of the foot. This variation is quite unusual.
Another unsual variation is Tsumasakigeri. If you are trained well, you can also kick using the tip of your toes. In Uechi Ryu they call this technique Sokusengeri.